1. Field of the Invention
This invention relates to an induction heating furnace and more particularly to an induction preheating furnace for selectively preheating certain portions of work objects prior to heating, in a second furnace, to a uniform temperature.
2. Description of the Prior Art
In many types of furnaces, the rate of heating of a work object is directly proportional to the surface area of the object and inversely proportional to the mass of the object. Furthermore, uniform heating of a work object is very desirable for various metallurgical properties; for example, grain size uniformity, normalizing, and annealing and hardness uniformity through quenching.
Thus, certain problems are encountered in uniformly heating irregularly shaped objects, such as upset tubing for example. Specifically, the more massive portions of the objects are heated to a lower temperature than the less massive portions of the object. In the case of upset tubing, the upset portion is heated to a lower temperature than the body of the tube. Thus, the prior art developed induction and gas preheating of the more massive portions of work objects, such as the upset portions of upset tubing, prior to the final heating step in a second (non-induction type) main heating furnace.
Still further, as is disclosed in U.S. Pat. No. 4,,093,839, granted to Moliterno, et al, selective induction heating of metallic tubing having a thickened portion, such as upset tubing, by selectively using high frequency and low frequency power sources, is known in the art.
Although the above-mentioned prior art has been successful in overcoming many of the problems in uniformly heating irregularly shaped objects, such as upset tubing, there remained the problem of non-uniform temperatures in different portions of the preheated sections, such as the upset section, after heating in a main heating furnace. In the case of upset tubing for example, the end of the upset tends to overheat in the main heating furnace because it has a larger surface area per unit of length of the upset, but an identical mass.
Furthermore, there remained the problem of higher temperatures, after final heating in a main heating furnace, in those portions of the work object immediately adjacent the preheated portions. Specifically, the end of the tube body connected to the upset tended to overheat in the main heating furnace because of some inadvertent preheating due to either conduction from the preheated upset portion or induction heating by the fringe magnetic field from the preheating induction coil.
Several solutions to the problem of non-uniform heating of irregularly shaped objects, such as upset tubing, have been proposed. In some applications a small gas-fired preheating furnace has been utilized. However, this has not solved any of the problems encountered in the prior art induction preheating furnaces.
Furthermore, it has been proposed to change the pitch (spacing of windings) of the preheating induction coil in order to selectively vary the intensity of the magnetic field produced by the coil. However, this solution has only been efective when irregularly shaped objects, all having the same size and shape, are preheated in a furnace specifically designed for preheating objects having that particular size and shape.
Thus, there has been a need in the art for a preheating furnace able to preheat certain portions of an irregularly shaped object, such as the upset portions of upset tubing, so that the entire object, including the entire preheated portion, will have a substantially uniform temperature after heating in a main heating furnace.
Furthermore, there has been a need for such a preheating furnace able to heat such portions having a wide variety of sizes, shapes and lengths; for example the upset portions of upset tubing, among others.